1. Field of the Invention
This invention relates to a master information carrier carrying thereon an irregularity pattern (a pattern of protruding portions and recessed portions) representing the information to be transferred to a magnetic medium.
2. Description of the Related Art
With an increase in information quantity, there is a demand for a magnetic recording medium which is high in memory capacity, low in cost and preferably requires a short time to read out a necessary part of data (a magnetic recording medium which allows so-called high-speed access). As an example of such a magnetic recording medium, there has been known a high recording density magnetic medium such as a hard disc, a zip (a flexible disc available from Iomega) and the like. In such a high recording density magnetic medium, the recording area is formed by narrow data tracks. In order to cause a magnetic head to accurately trace such narrow data tracks and reproduce the data at a high S/N ratio, the so-called servo tracking technique has been employed.
In order to perform the servo tracking, it is necessary to write servo information such as servo tracking signals for positioning the data tracks, address signals for the data tracks and reproduction clock signals on the magnetic recording medium as a preformat upon production thereof. At the present, such preformat recording is performed by the use of a specialized servo recording apparatus (a servo track writer). However, the preformat recording by the conventional servo recording apparatus is disadvantageous in that it takes a long time since the servo information must be recorded on the magnetic recording medium one by one by the use of a magnetic head, which deteriorates the productivity.
As a method of recording the preformat accurately and efficiently, there has been proposed, for instance, in Japanese Unexamined Patent Publication No. 63(1988)-183623 and U.S. Pat. No. 6,347,016, a magnetic transfer method in which a pattern which is formed on a master information carrier and represents servo information is copied to a magnetic recording medium (a slave medium) by magnetic transfer.
In the magnetic transfer, the magnetization pattern representing the information (e.g., servo information) carried by a master information carrier is magnetically transferred from the master information carrier to a slave medium by applying a transfer magnetic field to the slave medium and the master information in close contact with each other, and accordingly, the information carried by the master information carrier can be statically recorded on the slave medium with the relative position between the master information carrier and the slave medium kept constant. Thus, according to the magnetic transfer, the preformat recording can be performed accurately and the time required for the preformat recording is very short.
The master information carrier used in the magnetic transfer disclosed, for instance, in U.S. Pat. No. 6,347,016has an irregularity pattern (a pattern of protruding portions and recessed portions) representing information to be transferred to slave media and at least the top surface of the protruding portions of the irregularity pattern is formed of magnetic material. By magnetic transfer by the use of such a patterned master information carrier, a magnetization pattern corresponding to the irregularity pattern on the master information carrier is formed on the slave media.
In the magnetic transfer, the reproduction signal waveform obtained from the slave media to which the magnetization pattern is transferred from the master information carrier, that is, whether the reproduction signal waveform is of a desired amplitude and a desired period, is important.
Our, these inventor""s, investigation has revealed that the reproduction signal waveform from a magnetic recording medium obtained by the magnetic transfer depends upon the shape of the protruding portions of the irregularity pattern on the master information carrier, the strength of the transfer magnetic field, the distance between the master information carrier and the slave medium (the degree of close contact therebetween), and the like. The amplitude of the reproduction signal waveform is related to the signal quality (S/N), and a certain strength of the transfer magnetic field is required and the distance between the master information carrier and the slave medium should be as small as possible in order to obtain a good signal quality. Various shapes of the protruding portions of the irregularity pattern on the master information carrier have been proposed in view of using conditions such as on close contact, separation and the like, manufacturing conditions, and/or the like. In any shape of the protruding portions, there is an effective width in the direction of the tracks. However, it has not been clear what effective width in the direction of the tracks provides desired periodicity to the reproduction signal waveform.
Unless the reproduction signal waveform has desired periodicity, the recording/reproducing accuracy of the magnetic recording medium is deteriorated. Especially, when the information to be transferred is a servo signal, the tracking performance deteriorate, which can result in deterioration of reliability.
In view of the foregoing observations and description, the primary object of the present invention is to provide a master information carrier which can form a magnetization pattern on a magnetic recording medium by magnetic transfer so that a desired reproduction signal waveform can be obtained from the magnetic recording medium.
In accordance with the present invention, there is provided a master information carrier for magnetic transfer having on the surface thereof a concentric irregularity pattern representing information to be transferred to a disc-like magnetic recording medium, at least the top surface of the protruding portions of the irregularity pattern being formed of magnetic material, wherein the improvement comprises that
the irregularity pattern is formed so that the width L [nm] in a circumferential direction of a protruding portion at a distance R [nm] from the center satisfies the condition
L=xcex1(xcfx89xc2x7xcex94Txc2x7R), 0.8xe2x89xa6xcex1xe2x89xa61.4,
wherein xcfx89 represents in [rad/sec] the angular velocity of the magnetic recording medium when a signal is read out from the magnetic recording medium after the magnetic transfer and xcex94T represents in [sec] a desired signal time width of a reproduction signal waveform in a magnetization transition region corresponding to an interval between the ends of the protruding portion in the circumferential direction.
That is, in the master information carrier for magnetic transfer of the present invention, the width L [nm] in a circumferential direction (in a direction of track) of a protruding portion is changed according to the radius of the track (the distance from the center) with the value of xcex1 selected to an optimal value within the range of 0.8 to 1.4 according to the conditions such as the shape of the protruding portion, the strength of the magnetic field applied upon magnetic transfer, the distance between the master information carrier and the magnetic recording medium (the degree of close contact therebetween) and/or the like, so that the signal time width of the reproduction waveform reproduced at an angular velocity of xcfx89 is of a desired value in all the tracks.
The xe2x80x9cwidth in a circumferential direction of a protruding portionxe2x80x9d as used here means an effective width affecting formation of the magnetization pattern (the width of the magnetized area) by the protruding portion. For example, when the cross-sectional shape of the protruding portion taken along a plane extending in a direction parallel to the direction of the track in perpendicular to the surface of the master information carrier (will be simply referred to as xe2x80x9cthe longitudinal cross-sectional shapexe2x80x9d, hereinbelow) is a rectangle or trapezoid, the effective width is the width in the direction of the track of the top surface of the protruding portion, whereas when the longitudinal cross-sectional shape of the protruding portion is a rectangle or trapezoid with its upper portions cut away at the ends thereof, the effective width is the distance between the intersections of the extension of the upper side of the rectangle or the trapezoid and the extensions of the end lines of the rectangle or the trapezoid.
In the master information carrier of the present invention where the width L [nm] in a circumferential direction of a protruding portion at a distance R [nm] from the center satisfies the aforesaid condition, the reproduction signal waveform obtained from the slave media to which the magnetization pattern is transferred from the master information carrier can be of a desired amplitude and a desired period. Especially, when the information to be transferred is a servo signal, the accuracy of the tracking servo is improved.
When L/(xcfx89xc2x7xcex94Txc2x7R)(=xcex1) is smaller than 0.8, the longitudinal cross-sectional shape of the protruding portion must be a trapezoid having sufficiently inclined end lines in order to obtain desired periodicity of the reproduction signal, which makes it geometrically difficult to ensure a predetermined depth of recessed portion between protruding portions. Further when L/(xcfx89xc2x7xcex94Txc2x7R)(=xcex1) is larger than 1.4, the applied magnetic field must be strengthened or the distance between the master information carrier and the slave medium must be enlarged in order to obtain desired periodicity of the reproduction signal. However when the distance between the master information carrier and the slave medium is enlarged, the amplitude of the reproduction signal greatly deteriorates, that is, the signal quality greatly deteriorates.